According to Deuteronomy 15:1, what were Israelites to do at the end of every seven years?

Answer: Cancel debts

Every creditor was to cancel what he had loaned to his neighbor, because the LORD's time of release had been proclaimed.

“At the end of every seven years you must cancel debts. This is the manner of remission: Every creditor shall cancel what he has loaned to his neighbor. He is not to collect anything from his neighbor or brother, because the LORD’s time of release has been proclaimed.”— Deuteronomy 15:1-2 (BSB)